GL: fixed GL_DEBUG_* define name.

src/renderer_gl.cpp

@@ -2333,7 +2333,7 @@ namespace bgfx
 		if (s_extension[Extension::ARB_debug_output].m_supported)
 		{
 			GL_CHECK(glDebugMessageCallbackARB(debugProcCb, NULL) );
-			GL_CHECK(glDebugMessageControlARB(GL_DONT_CARE, GL_DONT_CARE, GL_DEBUG_SEVERITY_MEDIUM, 0, NULL, GL_TRUE) );
+			GL_CHECK(glDebugMessageControlARB(GL_DONT_CARE, GL_DONT_CARE, GL_DEBUG_SEVERITY_MEDIUM_ARB, 0, NULL, GL_TRUE) );
 		}
 #endif // BGFX_CONFIG_RENDERER_OPENGL
 	}